1. Technical Field
The invention is related to methods for forming ceramic matrix composites having ceramic fibers Coated with a toughening layer which facilitates fiber de-bonding and pull-out in the wake of a crack in the matrix.
2. Background Art
Fiber reinforced ceramic matrix composites comprise a weave of ceramic fibers embedded in a ceramic matrix. One way of improving the mechanical properties of such a composition is to provide a coating over the fibers which is stable and resistant to oxidation and which promotes fiber de-bonding at the tip of an advancing crack and fiber bridging and eventually pull-out in the wake of an advancing crack in the composite. This feature enhances the toughness, strength and strain to failure of the composite because the fibers remain mostly immune to advancing cracks in the matrix. The history of development of this technique is described in U.S. Pat. No. 4,885,199 to Corbin et al.
For certain applications, the ceramic matrix composite must be stable at temperatures above 2200 degrees F. in an oxidizing environment. Well-known fiber coatings such as carbon and boron nitride are not stable under such conditions. Any material to be substituted for a fiber coating in place of the carbon or boron nitride must be both resistant to oxidation and must possess sufficient strength to transfer loads from the matrix to the fiber while having a low shear strength to promote debonding between the fiber and the matrix in the presence of an advancing crack. Furthermore, the coating must be easy to apply to macrofibers, fiber tows and to a weave of ceramic fibers during manufacturing.
U.S. Pat. No. 4,935,387 and U.S. Pat. No. 4,948,758, both to Beall et al., disclose a sheet silicate coating on fibers which promotes fiber pull-out by cleavage failures between crystal sheets. These two patents by Beall et al. rely upon the nature of the crystalline cleavage or the inherent intrinsic bond between the silicate sheets to promote fiber pull-out. U.S. Pat. No. 4,405,685 to Honjo et al. describes a graded metal carbide coating on carbon fibers for isolating the fibers. U.S. Pat. No. 4,837,230 to Chen et al. discloses a sandwich structure of alternate layers of fiber reinforced composites formed by repeated impregnations with polycarbosilane, for example. U.S. Pat. No. 4,867,761 to Brandt et al. discloses coated whiskers in a whisker reinforced composite. U.S. Pat. No. 4,919,991 to Gadkaree discloses a continuous fiber reinforced glass matrix with particles such as SiC for matrix reinforcement.
The prior art as described in U.S. Pat. No. 4,885,199 referenced above typically relied upon the characteristics of the inherently weak shear strength of carbon and boron nitride coatings and a single weak interfacial bond between the fiber and coating to achieve desired characteristics, such as toughening. For applications in the high temperature oxidizing environments described above, the intrinsic properties of the coating composition would have to provide all of the necessary features, including fiber de-bonding and pull-out as well as imperviousness to oxidation and high temperatures. The problem with this approach is that it is very difficult to select the best fiber coating material for a given ceramic fiber so as to optimize all of the foregoing features in the same coating material composition.
Thus, one object of the present invention is to depart from the prior art approach of finding a coating composition which provides all of the necessary features, and instead find a mechanical approach in which all, or at least some, of the desired features (such as fiber de-bonding and pull-out in the wake of an advancing crack in the matrix) are realized through the mechanical features of the coating and coating/fiber interface, as distinguished from the inherent features of the composition. Such a mechanical approach has many advantages and, in most cases, allows greater choice in selecting the materials for use in the coating. For example, materials can be chosen to meet only the requirement of resistance to oxidation and stability at high temperatures, while the remaining requirements (e.g., fiber de-bonding and pull-out) are met by mechanical features in the coating or coating/fiber interface. In fact, the same composition as the fiber and/or the matrix would be a candidate for the fiber coating. These advantages will become clear in the description of the invention which follows the conclusion of this description of the background art.
In the present invention, the mechanical feature which promotes the requisite tendencies (e.g., fiber de-bonding and pull-out in the wake of an advancing crack in the fiber/matrix composite) is a fiber coating having thin multiple ceramic layers which are not bonded to one another or are weakly bonded and separable. Thus, the present invention, as will be described below, does not rely on the nature of the intrinsic properties of the coating material, the chemical bonds between the fiber coating and the fiber, nor upon the nature of the chemical bonds between the fiber coating and the matrix to promote fiber de-bonding and pull-out.
This contrasts with the technique disclosed in European Patent Publication No. 0 172 082 by Thebaut, which depends upon the existence of particular materials composed of weakly bonded layers (namely carbon or boron nitride) to promote fiber de-bonding and pull-out in the wake of an advancing crack in the matrix. Establishing such an intrinsic behavior severely limits the choice of materials, a significant disadvantage.
Accordingly, it is an object of the invention to provide a method for forming on a ceramic fiber a multi-layer coating whose layers are unbonded or separable and permit the fiber to de-bond and pull out from the coating under stress.
It is a further object of the invention to provide a ceramic fiber/ceramic matrix composite having a multi-layer coating covering the fibers, the multiple layers in the fiber coating being unbonded or separable from one another to promote fiber de-bonding at the leading tip of the crack and pull-out from the coating in the wake of the advancing crack in the ceramic matrix.
It is another object of the invention to provide a ceramic multi-layer coating for ceramic fibers which are to be immersed in a ceramic matrix, in which the ceramic coating comprises oxidation resistant oxide materials such as tantalum pentoxide, alumina or zirconia, or non-oxide materials such as silicon carbide or silicon nitride.
It is a related object of the invention to provide a method for inducing layering in a ceramic fiber coating deposited on ceramic fibers in which the fiber coating consists of oxidation resistant oxide or non-oxide ceramic materials having no inherent layering tendencies.
It is yet another object of the invention to provide a multi-layer ceramic fiber coating in accordance with the foregoing objects for a fiber/matrix composite in which the fiber and matrix consist of different ceramic materials and in which the multiple layers in the coating consist of different materials compatible with respective ones of the different fiber and matrix ceramic materials.